In achieving a compact high-efficiency motor, a permanent-magnet-excited motor using permanent magnets is most effective. General indexes of permanent-magnet-excited synchronous motor are introduced in Kazuo Onishi, “Investigation on Torque Evaluation and Optimum Structure of Permanent Magnet Motors”, The Transactions of the Institute of Electrical Engineering of Japan on Industry Applications, 1995, Volume 115-D, Number 7, pp. 930-935, which will be mentioned below.
Motors with cooling conditions equated and having the same size are assumed to be nearly equal in allowable dissipation Wc based on the relation between temperature rise and heat radiation. Torque T and allowable dissipation Wc are in the relation of Equation (1), and the coefficient Km is called a motor constant.T=Wc·√{square root over (Km)}  (1)
Accordingly, when the allowable dissipation Wc is constant, the torque T increases as the motor constant Km increases. The motor constant Km can therefore be used as an index value of allowable torque (usually, continuous rating torque).
The motor constant Km can be expressed by Equation (2). Herein, pole pair number p, winding maximum flux linkage Φ, space factor fs, total cross section St of winding slots, specific resistance ρ of winding, and average length l of unit coil are introduced. In addition, it is assumed that the current waveform is a sine wave, and magnetic fluxes alternate sinusoidally. Further, most part of loss in a motor, particularly when the motor is compact, is copper loss, and is considered omitting iron loss.Km=(½)pΦ√{square root over ((fsSt/ρl)}  (2)
Therefore, the motor constant Km needs to be increased in order to increase the motor efficiency per volume of motor, and the following measures are effective based on Equation (2):
(i) increasing the space factor fs of winding;
(ii) shortening the average length l of unit coil;
(iii) reducing the specific resistance ρ of winding;
(iv) increasing the winding maximum flux linkage Φ;
(v) increasing the pole pair number p; and
(vi) increasing the total cross section St of winding slot.
With regard to the measure (i), devices for slot configuration is proposed in Japanese Patent Application Laid-Open Nos. 2004-187370 and 2000-324728, for example. With regard to the measure (ii), it is achieved by shifting from the use of distributed winding to the use of concentrated winding. With regard to the measure (iii), the only existing material that has a specific resistance lower than copper is silver, which is not desirable costwise and industrially.
The measure (iv) involves employing a rare-earth magnet for permanent magnets, and in addition, increasing the surface area of pole face per unit volume of the motor. However, the increase in surface area of pole faces is not desirable from two points of view.
One of them lies in that it is desirable to employ an armature with windings wound therearound as a stator and to employ permanent magnets as field magnets in a rotor, and it is further desirable that the rotor be surrounded by the stator. Employment of the armature as the rotor requires a mechanical commutator for rectifying a winding current, which is not desirable in terms of high durability, high reliability, high dust resistance, and the like, so that it is desirable to constitute the rotor using permanent magnets as field magnets. Furthermore, it is desirable that a stator surrounding the rotor from outside be present in terms of inserting the motor into, for example, the inside of a compressor or the like. Then, the increase in surface area of pole face may become a cause of interfering with size reduction of the motor.
The other point of view also relates to the measure (vi). When increasing the surface area of pole face while leaving the outer diameter of the stator surrounding the rotor as it is in order to reduce the motor in size, the inner diameter of that stator also increases. This shortens slots of that stator in the radial direction, which reduces the total cross section St of winding slots. This is opposite to the measure desired in the measure (vi).
Further, in the case where the pole pair number p is increased on the basis of the measure (v), the total cross section St of winding slots is reduced.
On the other hand, as introduced in Research Specialized Committee of Higher Performance of Special-purpose-oriented Reluctance Torque Assisted Motor, “Higher Performance of Special-purpose-oriented Reluctance Torque Assisted Motor”, IEEJ Technical Report vol. 920, March 2003, which will be mentioned below, not only the magnet torque, but also the reluctance torque can be utilized in an interior permanent magnet type rotor in which field magnets are embedded in the inside of the rotor. Providing rotation angle dependency for the magnetic resistance of an iron portion of the rotor relative to the stator allows the armature current phase when energized to be shifted toward an advanced angle, and utilizing the reluctance torque generated from saliency of magnetic resistance increases the torque.
That is, introducing the pole pair number pn, flux linkage Φa, d-axis current Id, q-axis current Iq, d-axis inductance Ld and q-axis inductance Lq, the torque T is expressed by Equation (3).T=Pn(ΦaIq+(Ld−Lq)IdIq)  (3)
Similarly to the measures (iv) and (v), it is also desirable to increase the flux linkage Φa and pole pair number pn. However, increasing the q-axis inductance Lq further contributes to the increase in torque. This is because shifting the armature current phase toward an advanced angle makes the d-axis current Id negative.
On the other hand, since many of magnetic fluxes flowing to the rotor flow in the vicinity of its periphery, the total cross section St of winding slots can be increased when providing an armature also in the inside of the rotor. The technique of providing armatures inside and outside a rotor is introduced in, for example, Japanese Patent Application Laid-Open Nos. 2002-335658, 2002-369467, 2002-84720 and 9-56126.
It is however considered that the saliency of magnetic resistance cannot be utilized in the structure shown in Japanese Patent Application Laid-Open Nos. 2002-335658, 2002-369467 and 2002-84720, and in the structure shown in Japanese Patent Application Laid-Open No. 9-56126, an armature inside a rotor having field magnets is also a rotor, which is therefore considered difficult to make effective use of the reluctance torque of the rotor having the field magnets.
Japanese Patent Application Laid-Open No. 09-233887 proposes a technique of carrying out field weakening without flowing a field weakening current.
To increase the q-axis inductance Lq, it is possible to make the positions where permanent magnets are embedded close to the central axis of a rotor. This increases the volume of a rotor core positioned on outer side than the permanent magnets and increases the q-axis inductance Lq.
However, making the positions where permanent magnets are embedded close to the central axis of the rotor reduces the surface area of pole in the case where the rotor has a constant outer diameter, which contradicts the measure (iv). Further, it is difficult to employ the device of increasing the efficiency of a motor per volume by providing a stator also inside the rotor.